This invention relates to electronic key telephone systems (EKTS) and more particularly to those of the type enabling the user of any key telephone station set (TS) in the system to make different kinds of calls at the same time.
Previously known is an electronic key telephone system which uses the stored program type of control, including a plurality of telephone station sets each connected through two pairs of data transmission and reception lines and a pair of voice communication lines to a central key service unit including a central processor and space division communication paths employing electronic contacts. Such key telephone system is characteristically easy to install because of the extreme reduction in the number of cable conductors needed and has a variety of service functions (see the article "A Configuration of Key Telephone System" (in Japanese), by A. Miura et al, in the Journal of the Institute of Electronics and Communication Engineers of Japan, TGSE-1977-49).
However, key telephone systems of this known form transmit voice signals in the baseband, have often involved inconvenience to the telephone set users because only a single pair of voice communication lines are provided for each station set and hence there is a lack of such additional talking function as may be required for enabling the station set user to answer a second call while holding the first call connected to the station set.